1. Field of the Invention
The present invention relates to an optical film. In particular, the present invention relates to a diffusion film applicable to a backlight module.
2. Description of the Prior Art
Since liquid crystal panels cannot emit light, it is necessary to use a backlight module as a light source to offer sufficient and uniformly distributed light, so that the display device can display images normally.
Backlight modules can be substantially classified into two types, i.e., direct type and side type backlight modules. Direct type backlight modules have a light source disposed right below a diffusion plate and are generally utilized in a display device with a relatively large size, for example, TV sets. As for the side type backlight modules, the light source is disposed at the sides of the light guide plate, so that the light source emits light after being guided in a correct direction by the light guide plate. Generally, the side type backlight modules are applicable in a display device with a relatively small size, for example, notebook computers and monitors. However, both in the direct type and side type backlight modules, in order to eliminate the alternating bright-and-dark strips or mesh points, a diffusion film must be disposed above the light guide plate or the diffusion plate so as to uniformize the light, and to provide liquid crystal displays with a uniform surface light source.
Conventional diffusion films are mainly formed by applying a diffusion layer on a transparent substrate, the diffusion layer comprising a resin binder and chemical particles serving as diffusion particles. Once the light passes through the diffusion layer, the light will be refracted, reflected, and scattered because it passed through two media with different refractive indexes, so that the light can be effectively diffused, thereby achieving light uniformity. However, owing to the properties of the materials and the chemical particles involved, the light will be inevitably absorbed and scattered randomly, so that a portion of the light source is wasted and the light source cannot be efficiently utilized.
The diffusion particles normally used in the prior art have a particle size in the range of 1 μm to 50 μm. In order to enhance the light diffusion effect by increasing the coating area of the diffusion particles in the diffusion layer, it is known that particles with different particle sizes can be used as diffusion particles. As shown in FIG. 1, for a conventional diffusion film, a resin coating 103 containing a plurality of particles 105 with different particle sizes and a binder 104 is formed on a substrate 101. The diffusion particles used in the prior art have a wide particle size distribution. For example, when the particles used have a mean particle size of about 15 μm, the particle size distribution of the particles generally ranges from about 1 μm to about 30 μm. Although the light diffusion effect can be improved by using diffusion particles with different particle sizes, the light will be scattered randomly due to the different particle sizes of the particles, and as a result, the light source cannot be efficiently utilized.
It is known that, if the diffusion particles in the coating are aggregated or adhered to each other, not only is the light diffusion uniformity affected, but dark spots are also likely to be generated on the surface of the display. In order to solve the above problems, U.S. Pat. No. 7,218,450 B2 discloses using one or more organic or inorganic particles with a single distribution as diffusion particles with certain parameters, including the lamination ratio, particle size of the flocculated particles, and when two kinds of particles with a single distribution are used, the mean particle sizes of the two kinds of particles with a single distribution, that meet special formulae. 95% of the particles with a single distribution used in U.S. Pat. No. 7,218,450 B2 have a particle size ranging within ±15% of the mean particle size. U.S. Pat. No. 7,218,450 B2 further teaches using diffusion particles having a narrow particle size distribution, but is silent on the crosslinking degree of the diffusion particles. In fact, an insufficient crosslinking degree of the diffusion particles will inevitably cause some problems. For example, particles with a low crosslinking degree are likely to interact with the solvent in the binder and thus swell up. Therefore, compared with the particles with a high crosslinking degree, the particles with a low crosslinking degree will have a low solvent resistance. Furthermore, as the volume of the particles with a low crosslinking degree is changed because the solvent is absorbed, the optical properties of the particles become unstable, and the viscosity on the surfaces of the particles increases, so that the particles are easily aggregated with each other, thereby further affecting the coating processibility and light diffusion effects of the diffusion film.
In addition, in various optical films, the brightness enhancement film is relatively expensive, so in the newly developed backlight module structures, modifications were made to the other optical films and the combinations thereof so as to substitute for the brightness enhancement film and to reduce the cost. For example, in liquid crystal displays, two or three diffusion films were used to replace the conventional design of the brightness enhancement film with two diffusion films respectively located on and below the brightness enhancement film. Nevertheless, the brightness and the other performances are inferior compared with the conventional design. Therefore, for current techniques, the design of the diffusion film not only focuses on meeting the light diffusion efficiency requirement, but the means of improving the brightness of the diffusion film also needs to be considered.